Capacitively coupled collinear stripline antenna array



Jan. 9, 1962 E. G. FUBINI 3,016,536

CAFACITIVELY COUPL COLLINEAR STRIPLINE ANTE ARRAY Filed May 14, 1958 2 Sheets-Sheet 1 INVENT OR.

EUGENE G. FUBINI.

A TTORN K Jan. 9, 1962 E. G. FUBINI 3,016,536 CAPACIT LY COUPLED COLLINEAR STR INE ANTENNA ARRAY Filed May 14, 1958 2 Sheets-Sheet 2 FIG 4A 0 RELATIVE CURRENT I 8 6: N

DISTANCE ALONG ARRAY IN INCHES FIG. 5

20 A I a c a E a E a g Q Q Q Q 20- H -|9 g E E g H 22 20R guag v g g fl BlNl. 25V FIG. 6 BY ATTORNEY CAPACITWELY (IQUPLED COLLINEAR STRIP- LlZNE ANTENNA. ARRAY Eugene G. Fubini, Glen Head, N.Y., assignnr to the United tates of America as represented by the Secretary of the Army Filed May 14, W58, Ser. No. 735,344 4 Claims. (Cl. 343--3il1) tors disposed along a substantially straight line parallel to a reflecting ground plane. The radiators are separated from the ground plane and from one another by a dielectric medium. One of the radiators essentially comprises a center fed, half-wave antenna or half of a centerfed, full-wave antenna whereby, due to the capacitive coupling between radiators as a result of their separation, the entire array may be excited in phase.

The above-mentioned and other features and objects of the invention will become more apparent by reference to the following description and claims taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view of an antenna array constructed in accordance with the principles of the invention;

FIG. 2 is a sectional view taken along line 2-2 of FIG. 1;

I 'FIG. 3 is a diagrammatic representation of the voltage distribution along the antenna array of FIG. 1;

FIGS. 4A and 4B are graphs illustrative of the principles of the invention;

FIG. 5 is a top view of another antenna array constructed in accordance with the principles of the invention; and

FIG. 6 is a side view of a portion of the array of FIG. 5.

Referring now to FIGS. 1 and 2, there is shown an antenna array comprising, in collinear relationship, five electrical half-wave radiators separated from one another by capacitive gaps Ill]. The radiators comprise four electrical half-wave strip elements 31 and a half-wave antenna 12 consisting of two electrical quarter-wave strip elements 13. All of the strip elements 11 and 13 are bonded to or printed on a dielectric sheet 14. In turn,

' sheet 14 is supported by a substantially uniform layer of a low-dielectric-constant material 15 over a planar conductor 16 which serves as a reflecting ground plane. Antenna 12 is center fed from a source 17 of radio-frequency energy coupled to the adjacent ends of strip elements 13 through a parallel two-wire balanced transmission line 18. It is to be understood, of course, that insofar as the center feed apparatus is concerned, FIGS. 1 and 2 have been somewhat simplified for purposes of illustration, and that in most practical situations involving microwave frequencies a balun or other suitable device would be used to couple the energy of source 17 into line 18.

When radio-frequency energy from source 117 is applied to the array of FIG. 1, the well known voltage distribution of a center fed half-wave antenna is established along the length of antenna 12. In addition, I have found that by adjusting the capacitive coupling between adjacent Patented Jan. 9, 1962 ice half-wave strip elements 11 and the capacitive coupling between antenna 12 and the two half-wave strip elements adjacent thereto, e.g., by adjusting the length x of gaps 1i and the separation y of the planar conductor 16 from the dielectric sheet 14, it is possible to obtain a voltage distribution along the entire array which may be diagrammatically represented as shown in FIG. 3. With reference to FIG. 3, it is seen that there is a phase reversal at each of the gaps 10 whereby each of the elements 11 not only acts as a half-wave radiator, but also has a current distribution which is in phase with the current distributions of the remaining elements 11 and antenna 12. In other words, when the length x of gaps it and the distance y between planar conductor 16 and dielectric shcet 14 are such that conditions substantially as illustrated in FIG. 3 prevail, the array of FIGS. 1 and 2 yields a broadside radiation pattern. It has been found that for optimum results, the dimension of x should be substantially less than and the dimension of y should be in the neighborhood of a quarter-wavelength of the operating frequency.

Although the above-mentioned adjustment of the capacitive coupling between radiators has been described as being accomplished by adjusting the dimensions x and y shown in FIGS. 1 and 2 it is not to be understood that these are the only dimensions that can be altered to give a broadside radiation pattern. For example, the elements 11 may be slightly offset or canted at a slight angle with respect to one another and elements 13. In other words, the only restriction on the orientation of elements 11 and 13 in order that a broadside radiation pattern be obtained in accordance with the invention is that they be at least substantially collinear.

The side lobe level of the broadside radiation pattern produced by an antenna array constructed in accordance with the principles heretofore presented may be adequately limited by proper adjustment of the illumination of the half-wave radiators along the array. In general, it has been found that a change in the illumination along an antenna array constructed in accordance with the invention will produce a substantial change in the side lobe level without materially affecting the half-power beamwidth. The illumination along the array is adjusted, of course, by appropriately altering the capacitive coupling between the half-wave radiators consituting the array. For example, the half-power beamwidth of the array shown in FIGS. 1 and 2 may be comparable with the size of the apperture and yet the side lobe level may be too hi h due to the relative magnitude of the current in the half-wave strip elements at the extremities of the array being too high. This situation may be corrected by increasing the length x of the gaps 10 so that less energy will be coupled to these elements thereby causing a reduction in the side lobe level without producing any substantial change in the half-power beamwidth. FIG. 4A shows the relative magnitude of the current distribution of a typical antenna array constructed along the lines of FIG. 1 for which the illumination has been suitably tapered to yield a highly directional broadside pattern at 2900 megacycles. The E-plane radiation pattern of this array is shown in FIG. 43. It is, of course, understood that the adjustment of illumination herein referred to must be restricted to those distributions of current for which a complete phase reversal occur between the half-wave radiators as shown in FIG. 3. Otherwise, a good broadside pattern will not be produced. This is illustrated by FIG. 4 in which a current peak p at each of the gaps It indicates a corresponding complete reversal in the phase of the voltage along the array.

As an illustration of how the principles of the inven tion may be utilized to construct a large broadside array,

a printed microwave array comprising eight columns A, B, C, D, E, F, G and H of five collinear electrical halfwave radiators separated from one another by capacitive gaps 19 is shown in FIGS. and 6. The five half-Wave radiators in each column comprise four electrical halfwave strip elements 20 and a half-wave antenna consisting of two electrical quarter-wave strip elements 21. All of the strip elements 20 and 21 are printed on a sheet 22 of epoxy bonded =Fiberglas FF-91 or other suitable dielectric material. The dielectric sheet 22 is 21% inches in length, 4% inches in width, and 1 inch thick and is supported by a layer 25 of low-dielectric-constant material such as polyform inch over a planar conductor 23. Each column of strip elements is fed in phase from a parallel two-wire transmission line 241 printed on sheet 22 and connected to quarter-wave elements 21.. The radiation charcteristics of this array are given in Table 1 below over the frequency band from 2500 to 2700 megacylces. Also tabulated are the spacing between adjacent ones of the columns A, B, C, D, E, F, G, and H, the length of each of the gaps 19, and the dimensions of the elements 20, which are the same as the dimensions of the half-wave antennas formed by elements 21. It will be noted that the physical length of each of the elements 20 is approximately wavelength long at the tabulated operating frequencies although electrically each is approximately /2 wavelength along.

TABLEI Radiation characteristics of array Halt-Power Beam- Width (Degrees) Side-Lobe Level (DS) Frequency (me) E-Plane IEl-Plane E-Plane H-Plane 50 l3. 0 -19. l) 48 19. o 1s. 7 46 -14. 5

1 Smaller than.

Element Length=0.906 in. Element Width=0.228 in.

Gap Length=0.0l6 in.

Spacing Between Columns=3.0 in.

It is to be understood that the above-described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention. Moreover, while the invention has been described with reference to transmit ting antennas, it is to be understood, as will be obvious to anyone skilled in the art, that the invention is not limited thereto but may be applied as well to receiving antennas.

What is claimed is:

1. An antenna array comprising, in combination, a first electrical half-wave strip element, a second electrical halfwave strip element spaced from said first element and in substantially collinear relationship therewith, two discrete electrical quarter-wave strip elements spaced intermediate said half-wave element and substantially collinear therewith, means coupled to the adjacent ends of said quarterwave elements for propagating radio-frequency energy, a ground plane space from said quarter-wave and halfwave elements, and dielectric means intermediate said ground plane and elements for supporting said elements in substantially parallel relationship with said ground plane wherein the spacing of said strip elements from one another creates a capacitive coupling therebetween.

2. An antenna array comprising, in combination, a dielectric sheet of substantially uniform thickness, a pair of discrete substantially collinear electrical quarter-wave strip elements bonded to one face of said dielectric sheet, a like number of discrete electrical half-wave strip elements bonded to said one face of said dielectric sheet on either side of said pair of quarter-wave elements in substantially collinear relationship therewith, each of said quartenwave and half-wave strip elements being spaced from one another thereby forming a capacitive coupling between each pair of adjacent strip elements, a reflecting ground plane substantially parallel to and spaced from the other face of said dielectric sheet, a low-dielectric-constant material intermediate said ground plane and said dielectric sheet and means coupled to the adjacent ends of said quarter-wave elements for propagating radio-frequency energy.

3. An antenna array according to claim 1 wherein the distance between the ground plane and each of said radiators is of the order of a quarter-wave length at the operating frequency of said array and the spaces between adjacent ones of said radiators being substantially less than a quarter wave length at said operating frequency.

4-. An antenna array comprising, in combination, a plurality or electrical half-wave strip elements spaced from one another to create a capacitive coupling therebetween and arranged in substantially collinear relationship with one another, said plurality of halt-wave strip elements being further arranged into two groups, a pair of electrical quarter-Wavestrip elements capacitively spaced intermediate said groups of half-wave elements and in substantially collinear relationship therewith, a reflecting ground plane spaced from said quarter-wave and said half-wave elements, and means for supporting said quarter-wave and said half-wave elements in substantially parallel relationship with said ground plane.

References Cited in the file of this patent UNITED STATES PATENTS Franklin Tune 20, 1933 larns Dec. 30, 1952 OTHER REFERENCES UNITED STATESPATENT UFFIGE CERTIFICATE OF CORRECTIUN Patent; N0, 3 016 536 January 9 1962 Eugene Go Fubini It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3 line 13 for 'polyform". read me polyifoam a,

Signed and sealed this 5th day of June 19620 (SEAL) Attest:

ERNEST w, SWIDER DAVID LADD Attesting ficer Commissioner of Patents 

